In the food freezing industry, high food quality with low dehydration losses is obtained using low temperature liquid nitrogen freezing systems which operate at about -320.degree. F. Ammonia and freon vapor compression mechanical systems, which operate at relatively high temperatures, such as -40.degree. F., are commonly used to freeze food in an economical manner, but with high freezing times and high dehydration losses. Recently, high performance vapor compression, mechanical systems have emerged which produce high quality frozen foods with low dehydration losses at relatively high temperatures of from -40.degree. F. to -60.degree. F. Because they operate at such relatively high temperatures, dehydration losses associated with high performance mechanical freezers leave room for improvement. They typically also cannot operate at low temperatures due to limitations associated with common refrigerants. If a low temperature refrigerant system could be developed, dehydration losses can be appreciably reduced.
Direct contact reverse Brayton cycle, cold air refrigeration systems have been developed recently which operate at lower temperatures than mechanical systems. These systems cool food by generating cold air which directly impinges upon the food at high velocities. Cold air is created by compression/expansion and is then injected into the freezer. Air leaving the freezer is filtered to remove gross particulates and its refrigeration is recovered in a heat exchanger. The warmed air stream is then either vented or recycled back to the compressor, e.g. see U.S. Pat. No. 5,267,449 to Klezek, et al. Such systems are competitive with liquid nitrogen systems, based on operating cost, because they produce refrigeration at higher temperatures. However, their operating costs are higher than those associated with high performance mechanical freezers, even if improved dehydration losses are included in the analysis.
Power requirements associated with direct contact, reverse Brayton cycle refrigeration systems are high relative to mechanical systems for several reasons. At low air circulation rates, the air temperature rise across the freezer must be high to deliver sufficient refrigeration to cool the product. Because the freezer operates at atmospheric pressure, the pressure ratio across the compressor and turbine must therefore be large. As a result, power requirements are high. At high air circulation rates, the air temperature rise through the freezer is small and the pressure ratio across the compressor and turbine is small. However, since the freezer operates at atmospheric pressure, any pressure losses observed across, for example, filters and prepurifiers become significant relative to the operating pressure. Therefore, the power required is also high. A minimum power requirement exists where the combination of these two driving forces is minimized. This minimum is typically large relative to the power required for mechanical systems.
Several patents describe direct contact refrigeration systems wherein a refrigeration gas passes in direct contact with the product being frozen and is then recirculated, compressed, expanded and reused. Because those prior art systems are direct contact and apply the refrigerating gas at atmospheric pressure, filters, dehydrators, etc. are required in the return flow path to assure that entrained particulate matter and water do not cause undue deterioration of the refrigeration equipment. Such open loop systems can be found in the above noted Klezek et al. U.S. Pat. No. 5,267,449 and in the following U.S. Pat. Nos. 3,696,637 to Ness et al.; 3,868,827 to Linhardt et al.; 4,315,409 to Prentice et al.; 4,317,665 to Prentice; and 4,730,464 to Lotz.
Closed loop refrigeration systems have also been widely employed. Closed loop refrigeration systems operate with a primary refrigerant, generally at high pressure which is maintained in a closed path, with heat transfer being accomplished through a heat exchanger. For instance, such closed loop systems have been employed in gas liquefaction processes wherein the gas being liquefied takes one path through a heat exchanger and the primary refrigerant takes another independent path through the heat exchanger. Such systems are shown in U.S. Pat. Nos. 3,677,019 to Olszewski; 3,144,316 to Koehn et al.; and 4,778,497 to Hanson et al.
U.S. Pat. No. 3,696,637 to Ness et al. discloses apparatus for producing refrigeration that employs multiple stages of primary refrigerant compression and two stages of refrigerant work expansion in which the horsepower developed by the work expansion stages is utilized to drive the final stage of refrigerant compression.
It is an object of this invention to provide an improved refrigeration system which avoids subjecting a refrigerant gas that contacts a product being cooled to subsequent compression and expansion in a refrigeration cycle.
It is still another object of this invention to provide an improved refrigeration system wherein a principal refrigeration generation loop operates at high pressure, thereby lowering required power and enabling provision of smaller mass refrigeration components.